Why is the water that exits the anus considered impure in all circumstances, even though water may enter through the anus and exit purifying the impurity? Does this necessitate that the water remains impure even after the impurity has been removed?

It is sufficient to cleanse with the force of the water stream to remove impurities, and it is not necessary for the water to reach the inner part of the anus. The boundary of the inner part is what the hand cannot reach without inserting a finger into the anus, which is not legislated. Scholars have prohibited the burdening oneself with washing what is inner, due to the potential for obsessive whispering (waswasa) in that. Anything that water cannot reach is considered inner, and the inner excretions do not acquire the ruling of impurity until they emerge. And the water that is cut off after the removal of the impurity, if it exits unpolluted, is judged to be pure.